From David Friedman:

Re: More men than women in ancient African exodus, geneticists say (Dec. 21): Based on numerous studies I have read, I believe there is an alternatives explanation for tor the evidence found in the I chromosomes. If a small number of or even a single, matrilineal family group left Africa, perhaps in a search for greater resources or after losing a territorial battle with a more powerful group, a very narrow range of female DNA diversity would be present. With all females being closely related and most males coming from diverse backgrounds, over time the differences among X chromosome population would be minimized. This also seems consistent with mitochondrial DNA studies which seem to indicate a very narrow population left Africa in a first wave of migration.